rethrow semantics question

[yamt]

i'm reading https://github.com/WebAssembly/exception-handling/blob/main/proposals/exception-handling/Exceptions.md#rethrowing-an-exception how the following should work was not clear to me.

try
catch
  try
    try 
    catch
      rethrow 2
    end
  catch ;; can this catch the rethrown exception?
  end
  ;; or, does the above rethrow work as if it was "rethrow 0" here?
end

[ioannad]

i'm reading https://github.com/WebAssembly/exception-handling/blob/main/proposals/exception-handling/Exceptions.md#rethrowing-an-exception how the following should work was not clear to me.

try
catch x <---- the exception that this catch caught is rethrown below
  try
    try 
    catch
      rethrow 2
    end
  catch x ;; can this catch the rethrown exception? --> yes, if the tag indices match
  end
  ;; or, does the above rethrow work as if it was "rethrow 0" here? --> no
end

rethrow n throws from its position, basically this searches the caught exception a val* in the (n+1)th surrounding block, (here the 3rd) then pushes the caught exception's values val* onto the stack and throws the tag address a.

There is a detailed example of how this works in "formal-examples" document.

[aheejin]

This example may help understanding: https://github.com/WebAssembly/exception-handling/issues/146#issuecomment-777714491

[yamt]

thank you. so, in the following, N=1 and N=0 refer the same exception?

try
  throw $tag1
catch_all
  try
    try
      throw $tag2
    catch_all
      rethrow 2
    end
  catch_all
    rethrow N
  end
end

[aheejin]

Added the example to the explainer too: #263

[aheejin]

thank you. so, in the following, N=1 and N=0 refer the same exception?

No.

try
  throw $tag1
catch_all      ;; N == 1 will rethrow the exception caught by this
  try
    try
      throw $tag2
    catch_all
      rethrow 2
    end
  catch_all    ;; N == 0 will rethrow the exception caught by this
    rethrow N
  end
end

[yamt]

thank you. so, in the following, N=1 and N=0 refer the same exception?

No.

try
  throw $tag1
catch_all      ;; N == 1 will rethrow the exception caught by this
  try
    try
      throw $tag2
    catch_all
      rethrow 2
    end
  catch_all    ;; N == 0 will rethrow the exception caught by this
    rethrow N
  end
end

i don't understand.

if N==0,

1. "rethrow 2" rethrows the exception caught by the first catch_all
2. the exception is caught by the third catch_all
3. "rethrow N" rethrows the exception, which is the same exception "rethrow 2" rethrown.

if N==1,

1. same
2. same
3. "rethrow N" rethrows the exception caught by the first catch_all

so, in both cases, the exception thrown by "rethrow N" is the exception caught by the first catch_all.

what am i missing?

[aheejin]

Oh, you are talking about the program as a whole... Yeah, the whole program ends up throwing the same exception for N == 0 and N == 1. What I was describing was independent instruction's behavior.